You can use this simple function to filter bad words.

Please see attachment for mysql table of frequently used 458 bad words. http://www.phpmind.com/codedemo/php-bad-word-filter/bad_word.txt.zip
You can add more word easily as you know more!!

  function bad_wordcensor($txt)
  {
    $q = mysql_query("SELECT bad_word, replacement FROM bad_words");
    while ($row_bad = mysql_fetch_array($q))
    {
      $txt = str_ireplace($row_bad['bad_word'], $row_bad['replacement'], $txt);
    }

  return $txt;
  }

echo bad_wordcensor($_POST["comments"]);